A fluidized bed indirect gasification system may be comprised of two or more fluidized bed reactors which are separated into a gasifier and a combustor.
Conventional fluidized bed indirect gasification systems use steam as a gasifying agent and air as an oxidizing agent.
In particular, the gasifying agent and oxidizing agent may vary as necessary.
An indirect gasification system has a structure that a combustion gas is not mixed with a syngas generated in a gasifier because the system consists of a gasifier and a combustor, which are separated. Therefore, the syngas generated in the gasifier is not diluted with the combustion gas, and the system thus enables the production of syngas with high heating value.
The indirect gasification system requires a heat carrier that can transfer heat to a gasifier, where an endothermic reaction occurs, from a combustor.
For the heat transfer, a heat pipe or various other kinds of heat carriers may be used, and a bed material that is transported between the combustor and the gasifier serves the function of heat transfer in the case of a fluidized bed system.
The bed material performs heat transfer as it circulates through the combustor and the gasifier; the temperature of the bed material is increased to a high temperature in a combustor and the bed material is separated from the combustion gas via cyclone and supplied to the gasifier; the bed material, the temperature of which was decreased by a gasification reaction (i.e., an endothermic reaction), is again supplied to the combustor along with the unreacted carbon (char) remaining in the gasifier and burns the unreacted carbon; and the heat generated therefrom is again used to increase the temperature of the bed material, and the operation is performed as such.
In particular, an auxiliary fuel is provided to the combustor as necessary for temperature control.
Generally, low quality fuels, such as biomass/waste/coal, have various properties and they differ significantly with regard to physical characteristics, chemical characteristics, contents of impurities, etc.
Specifically, the factors that have the most significant effect on gasification and combustion may be the heating value, water content of a fuel, impurities contained in the fuel, environmental contamination-inducing materials as incombustibles such as ashes, stones, metals, glass, heavy metals, sulfur, chlorine, etc.
Among them, the operation problems due to incombustibles or impurities in a solid phase are ranked on the top.
Specifically, in the case of the waste among the various kinds of low quality fuels, it is essential to separate and sort out the incombustibles contained therein during the pre-treatment process.
In particular, the amount of incombustibles may vary significantly depending on the sorting process, and various incombustibles which are hard to separate depending on the sorting process are present. Even those fuels with a high impurity content may need to be used as a fuel if they contain at least a certain amount of combustible components.
In the case of fluidized bed indirect gasification systems, they differ in the degree of operation problems due to these impurities (incombustibles) according to the type of a gasifier or combustor, and specifically, the operation problem due to the incombustibles in solid phase accounts for the majority of the operation problems.
In the case of conventional indirect gasification systems, they have a fatal problem in that the impurities and inorganic materials contained in the fuels lower the melting point of bed materials and induce adhesion of bed materials at low temperatures, thereby causing an operation problem in the entire system.
Additionally, one of the most serious problems in the fluidized bed systems is the abrasion of reactor refractory by the bed materials, and in particular, the presence of a high content of incombustibles, such as metals, stones, glass, etc., in the low quality fuels may cause a serious damage on the inner wall of the reactor thereby reducing the lifecycle of a plant.
Meanwhile, in the case of the fluidized bed indirect gasification systems, where at least two reactors are required and the continuous transfer of heat and materials between the two reactors is important, there is a higher risk of the occurrence of the operation problem compared to the single fluidized bed.
Additionally, the fluidized bed indirect gasification systems have a problem in that the gasifier and the combustor have a very strong correlation with respect to heat and materials, and thus the optimal operation range for both reactors is very narrow.
Furthermore, in the case of conventional indirect gasification systems, they have a problem in that the occurrence of a problem in one of the reactors can cause the instability of the entire system due to the strong interaction between the two reactors.
(Patent Literature 1) Korean Patent Application Publication No. 2012-0124403